1. Field of the Invention
The present invention relates to an unpaved road detection system, wherein it can be determined whether the vehicle is traveling on unpaved roads including a gravel road and a dirt road, and an anti-skid control system having the unpaved road detection system, wherein brake pressure in each wheel brake cylinder is controlled on the basis of the result of the unpaved road detection system.
2. Description of the Related Arts
Various anti-skid control systems have been proposed heretofore to prevent a wheel from being locked during a braking operation of a vehicle. For example, a Japanese Patent Laid-open Publication No. 7-165053 discloses an anti-skid control system which is provided for estimating a desired slip rate, e.g., a slip rate at which a maximum coefficient of friction .mu. will be obtained, before the coefficient of friction .mu. of a road will become its maximum value in fact, and controlling the braking force applied to a wheel so as to equalize the actual coefficient of friction to the desired coefficient of friction. In practice, a slip rate is estimated when an acceleration of a wheel will exceed a predetermined value, then a peak of coefficient of friction is calculated on the basis of the wheel acceleration detected a predetermined time later, and then the slip rate at the peak of coefficient of friction is employed as the desired slip rate. In other words, when the variation of the slip rate is large when the acceleration of the wheel has exceeded the predetermined value, it can be estimated that the coefficient of friction will be of almost its peak value, so that the desired slip rate will be set to be of relatively small value, whereas when the variation of the slip rate is small, the desired slip rate will be set to be of relatively large value.
In general, the road for the vehicle can be classified into a smooth road or a rough road, which includes unpaved roads, stone pavement, snow road or the like. The unpaved roads include a gravel road and a dirt road. The dirt road is generally meant by a road of dried soil. The gravel road further includes a sand road, as far as the present invention is concerned. In order to distinguish these various kinds of roads, a detection system for classifying the roads into the smooth road or rough road has been already proposed and practiced. However, no system for determining the unpaved roads including the gravel road and dirt road has been known by now. According to the anti-skid control apparatus on the market, the locking condition of each wheel of the vehicle during the braking operation is controlled on the basis of the road conditions, i.e., smooth or rough. However, it has never been determined whether the vehicle is travelling on the unpaved roads, or not.
According to the anti-skid control system as disclosed in the aforementioned Japanese Patent Laid-open Publication No. 7-165053, the peak of coefficient of friction is estimated on the basis of the wheel acceleration before the coefficient of friction becomes its maximum value. When the vehicle vibrates, however, it is not easy to estimate the peak of coefficient of friction. Especially, on the unpaved roads such as the gravel road and dirt road, the coefficient of friction indicates such a characteristic that the coefficient of friction is gradually increased in response to increase of the slip rate. As a result, it is difficult to estimate the peak of the coefficient of friction, so that the slip rate determined to indicate the peak of the coefficient of friction will be of a relatively large value. Accordingly, it has been required to provide the system for determining the unpaved roads, and the anti-skid control system for controlling the brake pressure in accordance with the output of the unpaved road detection system.
With respect to the anti-skid control operation, it is preferable to provide the braking pressure control when the vehicle is traveling on the gravel road, and the braking pressure control when the vehicle is traveling on the dirt road, separately, rather than the braking pressure control for the unpaved road including the gravel road and dirt road. In the case where the braking pressure control is provided for the gravel road so as to reduce a braking distance, if the same braking pressure control is performed with respect to the dirt road, the vehicle stability might be deteriorated. Or, if the same braking pressure control for the dirt road is performed with respect to the gravel road, the braking distance might be prolonged. However, a system for distinguishing the gravel road from the dirt road has been unknown heretofore.